Impairment of Sympathetic Activation during Static Exercise in Patients with Muscle Phosphorylase Deficiency (McArdle's Disease) Susan L. Pryor,* Steven F. Lewis,* Ronald G. Haller,1 Loren A. Bertocci,11 and Ronald G. Victor* Departments of*Internal Medicine (Cardiology Division), tPhysiology, §Neurology, and I"Radiology, *the Harry S. Moss Heart Center, and "1the Biomedical Nuclear Magnetic Resonance Center, University of Texas Southwestern Medical Center, Dallas, Texas 75235; and §the Veterans Administration Medical Center, Dallas, Texas 75216

Abstract Static exercise in normal humans causes reflex increases in muscle sympathetic nerve activity (MSNA) that are closely coupled to the contraction-induced decrease in muscle cell pH, an index of glycogen degradation and glycolytic flux. To determine if sympathetic activation is attenuated when muscle glycogenolysis is blocked due to myophosphorylase deficiency (McArdle's disease), an inborn enzymatic defect localized to skeletal muscle, we now have performed microelectrode recordings of MSNA in four patients with McArdle's disease during static handgrip contraction. A level of static handgrip that more than doubled MSNA in normal humans had no effect on MSNA and caused an attenuated rise in blood pressure in the patients with myophosphorylase deficiency. In contrast, two nonexercise sympathetic stimuli, Valsalva's maneuver and cold pressor stimulation, evoked comparably large increases in MSNA in patients and normals. The principal new conclusion is that defective glycogen degradation in human skeletal muscle is associated with a specific reflex impairment in sympathetic activation during static exercise. (J. Clin. Invest. 1990. 85:1444-1449.) sympathetic nerve activity - microneurography * myophosphorylase deficiency * 31P nuclear magnetic resonance

spectroscopy

Introduction Static exercise causes increases in heart rate and blood pressure that are mediated by decreases in parasympathetic and increases in sympathetic neural activity (1, 2). These autonomic responses have been attributed both to neural signals arising in the central nervous system in association with voluntary motor effort, "central command" (3-6), and to reflexes caused by mechanically and chemically sensitive afferents in the contracting skeletal muscle (7-9). The relative contribution of these different mechanisms in the regulation of autonomic outflow during exercise may vary considerably depending on the experimental model and specific autonomic outflow under study (6, 10, 11). A preliminary report of this work was presented at the Annual Scientific Sessions of the American Heart Association (1987. Circulation. 76:IV-60.). Address correspondence to Dr. Ronald G. Victor, Cardiology Division, U. T. Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75235-9034. Received for publication 11 July 1989 and in revised form I November 1989.

J. Clin. Invest. © The American Society for Clinical Investigation, Inc.

0021-9738/90/05/1444/06 $2.00 Volume 85, May 1990, 1444-1449 1444

S.

Recent neurophysiologic studies in humans, for example, have provided substantial evidence that during moderate levels of static handgrip, stimulation of chemically sensitive muscle afferents is the principal mechanism responsible for increasing sympathetic outflow to resting skeletal muscle (10, 12). In contrast, central command and mechanically sensitive muscle afferents have no detectable effects on this particular sympathetic response (10, 13). The study of muscle sympathetic responses to static handgrip, therefore, provides an experimental approach to examine the function of chemically sensitive muscle afferents in conscious, exercising humans. We recently have combined this approach with phosphorus nuclear magnetic resonance spectroscopy (31P NMR)' in an attempt to probe the specific metabolic events in contracting skeletal muscle that initiate the reflex stimulation of sympathetic outflow evoked by static exercise. We found that during static handgrip in normal humans, the onset of sympathetic activation in resting leg muscle coincided with the contraction-induced fall in forearm muscle cell pH, an index of increased glycolytic flux resulting from glycogen degradation (12). The goal of the present study, therefore, was to test the hypothesis that glycogen degradation in exercising skeletal muscle plays an important role in initiating the reflex stimulation of sympathetic outflow evoked by static contraction. To test this hypothesis we performed microelectrode recordings of muscle sympathetic nerve activity (MSNA) during static exercise in patients with myophosphorylase deficiency (McArdle's disease), a condition in which contraction is not accompanied by glycogen degradation in exercising skeletal muscle ( 14-16).

Methods Four patients with myophosphorylase deficiency, ages 21-38 yr, participated in these studies. In each of the patients the diagnosis was established on the basis of a typical clinical history of exercise intolerance and exertional myoglobinuria, and muscle biopsy that showed absence of phosphorylase by histochemical and biochemical analysis. All four patients had completely normal neurological examination, 12-lead electrocardiograms, serum electrolytes, and hematocrits. One patient was taking L-thyroxine after complete thyroidectomy for a thyroid tumor in 1971; another patient was taking nadolol, which was discontinued 72 h before the study. Nine healthy volunteers matched for age, sex, and maximal handgrip strength served as control subjects. The study protocol was approved by the Institutional Review Board of human investigation and each subject and patient gave informed written consent to participate. Multiunit recordings of efferent sympathetic nerve activity were obtained with a unipolar tungsten microelectrode inserted into a mus-

1. Abbreviations used in this paper: MSNA, muscle sympathetic nerve activity; MVC, maximal voluntary contraction; NMR, nuclear magnetic resonance; PCr, phosphocreatine; Pi, inorganic phosphate.

L. Pryor, S. F. Lewis, R. G. Haller, L. A. Bertocci, and R. G. Victor

cle fascicle of the right peroneal nerve posterior to the fibular head by microneurography. The details of this technique have been previously described (17). Postganglionic action potentials were amplified 20,000-50,000-fold, filtered using a band width of 700-2,000 Hz, rectified, and integrated (time constant, 0.1 s) to obtain a mean voltage display of MSNA. An acceptable recording site was identified by neurograms that demonstrated spontaneous pulse-synchronized bursts which increased during expiration and during phases 2 and 3 of a Valsalva maneuver, but not during arousal stimuli (loud noise, skin pinch) (17). Sympathetic bursts were identified by inspection of the filtered and mean voltage neurograms. Simultaneous measurements of MSNA, arterial pressure (oscillometric sphygmomanometer; Critikon, Inc., Tampa, FL), heart rate (electrocardiogram), and force of contraction (handgrip dynamometer; Stoelting Co., Chicago, IL) were performed during a 2-min control period, followed by 90 s of static handgrip at 30% of maximal voluntary contraction (MVC) and a 2-min recovery period. All measurements were obtained with the subjects and patients in the supine position. In eight additional experiments, we monitored cellular high energy phosphates and pH in exercising forearm muscle with 31P NMR spectroscopy. The handgrip protocol was repeated on a separate day in seven normal subjects and one patient with myophosphorylase deficiency with the exercising forearm placed in a 30-cm horizontal bore, 1.9-tesla superconducting magnet (Oxford Instruments, Oxford, UK) interfaced to a NT-80 console (GE/NMR, Freemont, CA). A 2-cm surface coil tuned to the resonance frequency of 32.5 mHz for 31p was positioned over the flexor digitorium profundis muscle. Magnetic resonance spectra were acquired in 30-s intervals, representing the time average of 20 acquisitions. The cellular concentrations of phosphocreatine (PCr) and inorganic phosphate (Pi) were measured by calculating the area of their respective peaks. Intracellular pH was estimated from the chemical shift of Pi relative to PCr (18). Free intracellular ADP was calculated using the creatine kinase equilibrium reaction, assuming an equilibrium constant of 5.5 mmol/kg for ATP and 32 mmol/kg for PCr + Cr (19). We also examined MSNA responses evoked by baroreceptor deactivation during the Valsalva maneuver and by stimulation of cutaneous afferents during the cold pressor test (hand in ice water for 2 min). These two reflex interventions were used as internal controls, i.e., as nonexercise stimuli to sympathetic outflow (17, 20). The order of interventions was randomized with 10-min rest periods between interventions. Statistical analysis was performed using repeated measures analysis of variance with the Bonferroni adjustment for multiple comparisons. Differences were considered statistically significant for P < 0.05. Data are expressed as mean±SE.

Results Resting NMR spectra, peroneal neurograms, blood pressures, and heart rates all were normal in patients with myophosphorylase deficiency (Tables I and II, Figs. 1 and 2). In both patients and normal control subjects, resting neurograms revealed pulse-synchronous bursts of muscle sympathetic activity. At rest, the sympathetic nerves fired more frequently in the patients vs. controls: 36±6 vs. 23±4 bursts/min (P < 0.05). However, in two of the nine control subjects the resting level of sympathetic discharge was 2 40 bursts/min, a level that was equivalent to that in the two patients with the highest levels of resting sympathetic discharge. Maximal handgrip strength was 39±6 kg in the normal subjects and 36±10 kg in the patients. In normal subjects, static handgrip at 30% MVC decreased PCr and increased Pi and ADP, decreased pH, and increased MSNA by 134% (Tables I and II, Figs. 1 and 2). Increases in MSNA were temporally correlated with decreases in muscle cell pH, beginning 30-60 s from the onset of tension development. In contrast, in one patient with myophosphorylase defi-

Table I. Effects of Static Handgrip on Muscle Cell pH and High Energy Phosphates Patient with

myophosphorylase deficiency

Normal subjects

[H+] (nM) pH

[PCr] (mmol/kg wet wt) [Pi] (mmol/kg wet wt) [ADP] (gmol/kg wet wt)

Control

Handgrip

Control

Handgrip

79.4±0.3 7.1±0.1 22.9±0.3

126±1.2 6.9±0.1 14.7±1.1 10.9±1.2 27.4±6.1

85.1 7.1 23.8 4.8 8.2

77.6 7.1 8.3 22.5 86.9

2.6±0.2 11.4±1.1

Data are mean±SE for seven normal subjects and one patient with myophosphorylase deficiency. Entries are the average of 2 min of control measurements and the last 30 s of a 90-s static handgrip at 30% MVC.

ciency in whom 31P NMR spectroscopy was performed, handgrip had no effect on pH but caused excessive decreases in PCr and increases in Pi and ADP (Table I, Fig. 1). Most importantly, even though force production was comparable in the two groups, handgrip caused no increase in MSNA in any of the four patients with myophosphorylase deficiency, even though this maneuver more than doubled MSNA in the normal subjects (Table II, Figs. 2 and 3). The exercise-induced rise in blood pressure also was greatly attenuated in the patients, but exercise-induced increases in heart rate were comparable NORMAL SUBJECT P*r

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Figure 1. Illustrative 31 P NMR spectra at rest and during static handgrip at 30% of maximal voluntary contraction (MVC) in two subjects. Data represent the last 30 s of each measurement period. In each spectra, the signal intensities are proportional to the cellular concentrations of Pi, PCr, and the three phosphates of ATP. In the normal subject, handgrip increased Pi as PCr decreased, and decreased pH (estimated from rightward shift of Pi relative to PCr). In the patient with myophosphorylase deficiency, despite an excessive increase in Pi and decrease in PCr, handgrip caused no decrease in pH.

Sympathetic Activation in McArdle's Disease

1445

Table II. Responses to Static Handgrip at 30% MVC Static handgrip Control

0-30 s

30-60 s

60-90 s

Recovery

304±48 23±4 89±2 65±3

287±38 22±4 95±3§ 71±3§ 12±2

502±76§ 34±6§ 101±3§ 76±3§

700±105§ 41±5§ 110±3§

400±52 28±5 89±3 66±3

12±2

12±2

488 38 88 78

435 38 91 90 5

459 40 87 74

5

380 30 91 80 5

475 34 98 82 7

534 40 99 85 7

464 36 99 88 7

500

686 46 91 71

649 41 95 75 17

739 45 96 78 17

739 45 88 67

112 13 87 67 17

217 22 92 70 17

181 19 84 57

419±116 31±6

464±107 35±5 95±2§ 82±5§ 12±3

470±114 36±6 89±3 70±5

Normal subjects*

MSNA (total activity)t MSNA (bursts/min) MAP (mmHg) HR (beats/min) Force (kg) Patients with myophosphorylase deficiency" Patient 1 MSNA (total activity)t MSNA (bursts/min) MAP (mmHg) HR (beats/min) Force (kg) Patient 2 MSNA (total activity)t MSNA (bursts/min) MAP (mmHg) HR (beats/min) Force (kg) Patient 3 MSNA (total activity)t MSNA (bursts/min) MAP (mmHg) HR (beats/min) Force (kg) Patient 4 MSNA (total activity)t MSNA (bursts/min) MAP (mmHg) HR (beats/min) Force (kg) Mean±SE MSNA (total activity)t MSNA (bursts/min) MAP (mmHg) HR (beats/min) Force (kg)

531 38 85 71

560 44 97

81 682 44

88 68

17

166 18 85 55

85 10 85 61 17

485±111 36±6 89±3 69±5

434± 126 32±8 91±3 73±5 12±3

93±3§ 77±4§ 12±3

80±3§

41 97

82

* Entries are mean±SE for nine subjects. * MSNA is expressed as total activity (bursts/minute X mean burst amplitude). § P < 0.05 vs. control values. 1 Entries are data for MSNA, MAP, HR and force during static handgrip at 30% MVC in four patients with myophosphorylase deficiency.

in the two groups (Table II, Fig. 3). At the end of handgrip exercise, three of the four patients developed forearm contractures lasting 10-20 min. This contracture caused pain, but no increase in MSNA, blood pressure, or heart rate. Unlike handgrip, which had no effect on MSNA in patients with myophosphorylase deficiency, two nonexercise stimuli to sympathetic outflow, the Valsalva maneuver and the cold pressor test, evoked comparably large increases in MSNA in patients and normal subjects (Table III, Figs. 4 and 5).

Discussion We performed direct measurements of sympathetic nerve discharge in humans with a specific inborn error of skeletal mus1446

cle metabolism to examine the relationship between muscle metabolism and sympathetic outflow during static exercise. The major new finding is that a level of static handgrip that consistently elicits large increases in muscle sympathetic outflow in normal humans has no effect on this sympathetic outflow in patients with myophosphorylase deficiency. This observation strongly suggests that without glycogen degradation in contracting muscle static exercise alone is not sufficient to stimulate muscle sympathetic outflow in humans. This interpretation is predicated on the assumption that the enzymatic defect in these patients is limited to skeletal muscle and does not involve afferent, central, and efferent neural pathways. There is substantial evidence to suggest that skeletal muscle phosphorylase deficiency is not associated with

S. L. Pryor, S. F. Lewis, R. G. Haller, L. A. Bertocci, and R. G. Victor

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Figure 2. Segments of original records from one normal subject and one patient with myophosphorylase deficiency showing recordings of MSNA during 90 s of static handgrip at 30% of MVC. Static handgrip markedly increased the frequency and amplitude of the sympathetic neural bursts in the normal subject but had no effect on MSNA in the patient with myophosphorylase deficiency despite comparable force production.

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abnormal phosphorylase activity in either cardiac or vascular smooth muscle (21). In our patients, preservation of reflex sympathetic activation during Valsalva's maneuver and during cold pressor stimulation strongly suggests that efferent sympathetic pathways were intact and could respond appropriately to deactivation of baroreceptors and to activation of somatic afferents in skin. The additional finding that our patients experienced muscular pain during handgrip-induced contracture of the forearm flexor muscles indicates that somatic afferents, presumably unmyelinated afferents, were also intact in skeletal muscle. Unmyelinated fibers are the subtype of afferents that, in anesthetized animal preparations, are stimulated not only by algesic substances, but also by chemical products of muscle contraction (7). Because the resting frequency of sympathetic discharge was on the average higher in the patients than in our control subjects, we considered the possibility that in the patients with the highest levels of resting sympathetic discharge, MSNA already was maximal at rest and thus could not possibly have increased further with static handgrip. This possibility is unlikely for two reasons. First, two of our nine normal control subjects had resting levels of nerve traffic that were 2 40 bursts/min, levels that were just as high as those seen in the two patients A

MAP (mm HN)

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200

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